The CDKs are a family of serine/threonine protein kinases that are classified as regulators of the cell cycle (CDK1, 2, 4, 6) or transcription (CDK7, 8, 9, 11, 20). However, in the last years they have been shown to participate also in angiogenesis, senescence, exocytosis, spermatogenesis, and neuronal development (Malumbres M. Genome Biol 2014; 15:122). CDK activity is dependent on the binding of regulatory subunits called cyclins, which are produced and degraded during different phases of the cell cycle. The timing of CDK activity is also subject to negative regulation mediated by the binding of natural CDK inhibitors (INK4, Cip/Kip), and by inhibitory phosphorylation catalyzed by the Weel and Myt1 kinases, which can be reversed by the cdc25 phosphatases (Pavletich N P, J Mol Biol 1999; 287:821-828; Boutros and Duccomun, Cell Cycle 2008; 7:401-406).
The uncontrolled upregulation of CDK activity has been identified as a hallmark of cancer and there are several mechanisms described to cause CDK hyperactivity. Loss of function mutations (deletions, silencing, or point mutations) affecting natural CDK inhibitors as well as an overexpression of CDK-activating cyclins are common ways of such deregulation. Besides that, an excessive production of cyclin D1 was detected in breast, bladder, esophageal and squamous cell carcinoma, cyclin E in colon, lung and breast cancers and in some leukemias, or cyclin A in lung carcinoma (Hall and Peters, Adv Cancer Res 1996; 68:67-108; Leach et al, Cancer Res 1993; 53:1986-1989; Dobashi et al, Am J Pathol 1998; 153:963-972; Keyomarsi et al, Oncogene 1995; 11:941-950; Iida et al, Blood 1997; 90:3707-3713). However, in some cases, especially those involving CDK4 and 6, hyperactivity is caused by the amplification or overexpression of the CDK gene itself (Nagel et al, Leukemia 2008; 22:387-392, Faussillon et al, Cancer Lett 2005; 221:67-75, Tang et al., Clin Cancer Res 2012; 18:4612-4620).
Alternatively, non-mitotic CDKs have been also found to be implicated in cellular transformation and/or cancer phenotype. Notable example is CDK9, a kinase that regulates elongational phase of mRNA transcription by phosphorylation of C-terminus of RNA polymerase II. It has been well documented that many cancer types, including leukemias, are heavily dependent on continuous expression of anti-apoptotic and pro-survival genes such as Mcl-1 or survivin (Chen et al, Blood 2005, 106, 2513; McMillin et al, Br. J. Haematol. 2011, 152, 420). Another example is CDK5, a kinase that regulates cellular migration; it has been found to be hyperactivated in some human cancers and promotes their metastasis (Eggers et al, Clin Cancer Res. 2011; 17(19):6140-5). In addition, CDK5 is highly expressed and active in proliferating endothelial cells (J Cell Biochem. 2004; 91(2): 398-409) and its function in these cells is linked to their migration and sprouting and promotes angiogenesis (Liebl et al, J Biol Chem. 2010; 285(46):35932-43).
For these reasons, CDKs and their natural binding partners have become important targets for anticancer drug development. Most medicinal programmes in this area have focused on small molecule inhibitors. Most known CDK inhibitors are pan-selective and they block the transcriptional regulators CDK7 and CDK9 in addition to the cell cycle regulating CDK1, CDK2 and CDK4. These compounds induce cell cycle arrest and activate apoptosis by inhibiting transcription, which is most effective in cells that are strongly dependent on the expression of antiapoptotic proteins with short half-lives such as Mcl-1. Many groups have demonstrated that early inhibitors such as roscovitine and flavopiridol are effective against multiple myeloma and other malignancies that depend on continuous mRNA synthesis and Mcl-1 expression (Raje et al, Blood 2005; 106:1042-1047; Gojo et al, Clin Cancer Res 2002; 8:3527-3538). Inhibitors of the transcriptional CDKs also influence the stabilization of the tumor suppressor p53, probably by downregulating its target genes; these include the ubiquitin ligase Mdm2, which negatively regulates p53 (Dai and Lu, J Biol Chem 2004; 279:44475-44482). Based on in vitro studies, simultaneous inhibition of several CDKs (i.e. CDK1, 2 and 9) has been proposed as a viable strategy for selecting clinical drug candidates (Cai et al, Cancer Res 2006; 66:9270-9280).
It has been shown that inhibition of CDK5 results in the suppression of cancer growth and metastatic progression in preclinical models of pancreatic and breast cancer (Feldman et al, Cancer Res. 2010; 70(11):4460-9; Feldman et al, Cancer Biol Ther. 2011; 12(7):598-609; Liang et al, Sci Rep. 2013; 3:2932). At the molecular level, CDK5 is essential for TGF-b-induced epithelial-mesenchymal transition and breast cancer progression (Liang et al, Sci Rep. 2013; 3:2932). In addition, inhibition of CDK5 in endothelial cells also suppresses angiogenesis (Liebl et al., J Biol Chem. 2010; 285(46):35932-43; Liebl et al., Angiogenesis. 2011; 14(3):281-91). In leukemic cells, CDK5 has been found to phosphorylate Noxa, a BH3-only member of the Bcl-2 family, and its inhibition promoted apoptosis (Lowman et al, Mol Cell 2010; 40(5):823-33).
It is an object of this invention to provide new, very potent anticancer, antileukemic, and antiangiogenic compounds.